Certain N-nitrosamines (hereinafter abbreviated to "nitrosamines") are carcinogenic, and a problem arises in that it is possible that nitrosamines are produced in vulcanized rubber articles made from a solid polymer rubber or a polymer rubber latex.
For example, as examples of the vulcanized rubber articles made from a polymer rubber latex, there can be mentioned those which are used in contact with the human body, such as a nipple, a balloon, gloves for operation, or medical examination or detection, a balloon sac, and other medical articles. It is possible that nitrosamines are detected in these vulcanized rubber articles. Especially a serious problem is caused in medical rubber articles which are used in direct contact with mucous membranes or organs.
At present, the restriction of nitrosamines contained in vulcanized rubber articles by regulation is an urgent problem in Japan and many other countries. Already in German, the maximum permissible amount of specific nitrosamines is 10 ppb or below. As such nitrosamines which are restricted by regulation, there can be mentioned seven species, which include, for example, N-nitrosodimethylamine, N-nitrosodiethylamine, N-nitrosodi-n-butylamine and N-nitrosomethylphenylamine.
It is known that the occurrence of nitrosamines in vulcanized rubber articles is due to the fact that a dithiocarbamic acid compound used as a polymerization terminator for emulsion polymerization or as a vulcanization promoter emulsifier remains in a solid polymer rubber or polymer rubber latex. Namely, the dithiocarbamic acid compound is hydrolyzed to produce a secondary amine which in turn reacts with NO.sub.x in the environment or nitrites or other NO.sub.x contained in food or the saliva to produce nitrosamines.
Thus it has been studied to use dithiocarbamic acid compounds as a vulcanization accelerator for rubber, which produce only a negligible amount of a nitrosamine or a secondary amine, i.e., a precursor of nitrosamine. As an example of such dithiocarbamic acid compounds, there can be mentioned zinc dibenzyldithiocarbamate. However, it is reported that when this vulcanization accelerator is used for a natural rubber latex, the resulting vulcanized rubbers generally have poor vulcanization properties as compared with vulcanized rubbers made with other conventional vulcanization accelerators (Polymer Digest, 1987, 12, p12-). The present inventors have also confirmed that when zinc dithiocarbamates are incorporated in a natural rubber latex, rubber articles dip-formed from the natural rubber latex have cracks and poor surface luster.
A vulcanization accelerator incapable of producing a nitrosamine, which is different from the dithiocarbamic acid compound vulcanization accelerator, such as zinc isopropylxanthogenate, has also been reported. This vulcanization accelerator is however little or no practicality because it has a low storage stability and an offensive smell, and results in rubber vulcanizates with poor vulcanization properties.
Other types of vulcanization accelerators such as thiophosphate compounds, thiazole compounds, benzothiazole-sulphenamide compounds and guanidine compounds are known, but desired vulcanization properties cannot be obtained with these vulcanization accelerators (Polymer Digest, 1991, 1, p65-).
Thus there is an increasing demand for a vulcanization accelerator which is incapable of producing a nitrosamine or a secondary amine and giving a vulcanized rubber having good vulcanization properties.